Stylus pointing devices enable information to be input to a host electronic device. When the tip of a stylus is placed in close proximity to a display surface of the host device, the position of the tip may be determined by the host by a variety of methods, including the effect of the stylus on the electrical properties of the tablet (i.e., via electromagnetic induction, changes in electrical resistance, electrical capacitance, and the like); the optical properties of the tablet; or by ultrasonic positioning.
One method for determining stylus location is to employ a grid of overlapping coils embedded in a surface of the host to sense an electromagnetic field generated by a transmitter in the stylus. Typically, one set of elongated coils extends in the horizontal (X) direction and another set of elongated coils extends in the vertical (Y) direction. The sensed electromagnetic field information is processed to yield a location.
Another method energizes the X- and Y-coils of the grid sequentially and senses the resulting voltage induced in a receiving coil of the stylus.
One disadvantage of these approaches is that a large number of coils is required to provide accurate location information.
A magnetometer is a sensing device that produces an output signal, such as voltage signal, in response to an applied magnetic field. Commonly, such devices utilize a Hall sensor, which senses the voltage difference produced across a current-carrying conductor in the presence of a magnetic field, or a magnetostrictive sensor. Magnetometers are responsive to the earth's magnetic field and may be employed as compasses.
The earth's magnetic field is altered by the presence of ferrous objects. Accordingly, variations in the earth's magnetic field may be employed to detect ferrous objects by moving one or more magnetometers across a region.
A single compass embedded in an electronic device may be employed to detect changes in a magnetic field caused by movement of a magnetic object, such as a magnetic ring or pen, around the device. This approach senses motion and may be used for gesture detection, but cannot be employed to sense location due to the interfering presence of magnetic fields produced by the electronic device and by the earth's magnetic field and because the magnetic field is dependent upon the orientation of the magnetic object with respect to the electronic device.
It would be useful to provide an alternative way to accurately sense the location of a stylus with respect to a host electronic device.